The following information may be out-of-date or incorrect; some of it is based on rumors and I haven't personally verified all of it.
facebook
This site is one of the worst privacy offenders; every year there seems to be a new opt-out giveaway of your personal information, so that you have to keep on top of it and keep logging in and changing your setting to opt-out of the new stuff.
- no matter what you do, facebook will share your name, profile picture, and "connection" list with everybody, even Google and other entities not logged into facebook
- "When you visit a Facebook-enhanced application or website, it may access any information you have made visible to Everyone (Edit Profile Privacy) as well as your publicly available information. This includes your Name, Profile Picture, Gender, Current City, Networks, Friend List, and Pages. The application will request your permission to access any additional information it needs."
- Apparently also "interests, "likes," work, education, current city and hometown"
- go to http://www.facebook.com/settings/?tab=privacy and make thing more private
- Go to Settings->Application Settings, set the spinner to Authorized, remove all the applications that you don't want to have permanent access to your information More on Facebook's attitude: http://www.readwriteweb.com/archives/facebooks_zuckerberg_says_the_age_of_privacy_is_ov.php
google
- If you have a Google Profile, go to http://www.google.com/profiles/me/editprofile and make sure "Display the list of people I'm following and people following me" is unchecked (otherwise people can see your Google "friend list"
- Don't check the Custom URL checkbox, because doing so will allow spammers to find you in a websearch and then acquire your Google Email address
identi.ca, twitter